In recent years, microprocessor controlled drug infusion systems have become relatively common. A typical control for a drug delivery system includes a keypad or other user interface enabling a medical practitioner to enter the rate of drug delivery, duration, and volume of a drug or medicinal fluid to be infused into a patient. Drug delivery is normally either programmed to occur as a continuous infusion or as a single bolus dose.
It is not uncommon for a plurality of different drugs to be prescribed for intravascular delivery to a patient, either in a sequence, or simultaneously. Although two or more pumps can be employed to infuse a plurality of drugs, there are several clear advantages to using a single multi-channel pump for this purpose. Simply minimizing the number of separate drug delivery pumps and the concomitant hardware that must be connected to the patient in order to simultaneously administer several different drugs is probably adequate justification for use of a multi-channel drug infusion system, but there are other more important advantages. For example, certain drugs are chemically incompatible. The controller of a multi-channel drug delivery system can be programmed to flush a common drug delivery line with a saline fluid between infusion of two incompatible drugs and to warn medical personnel of the potential problem if simultaneous infusion of the two drugs is attempted. It is also much easier to program a complete drug delivery regimen on a single control instead of programming several different drug delivery systems.
Although conventional drug infusion controllers have greatly improved the efficiency and ease with which drugs are delivered to a patient, medical personnel must still enter the rate and volume that is appropriate for the specific drug to be infused, to achieve a desired effect. In many cases, the physician will want to quickly achieve a desired blood plasma concentration of a drug and then maintain that concentration for a defined period of time. The blood plasma concentration that will result from a given rate of drug infusion depends upon certain characteristics of the patient, such as age, weight, and gender. Accordingly, the physician will typically rely upon prior experience with a drug or be forced to consult reference materials to determine the rate at which a drug should be delivered. Initially, the physician may program the drug infusion system to deliver a bolus of the drug, followed by a continuous infusion at a lower rate, and then, may adjust the delivery rate based upon the apparent effect of the drug on the patient. With considerable experience, a physician may be able to accurately estimate the proper rate settings to achieve the desired effect by a particular drug. However, in many cases, the physician's initial estimate of the appropriate rate of drug delivery will need to be modified several times, based upon the observed effect of the drug on the patient. Given the computational power of the microprocessors typically used for controlling an infusion system, it should be possible for the control to automatically determine the rate of drug infusion to achieve a desired drug concentration.
U.S. Pat. No. 5,010,473 (Jacobs) discloses a model based open-loop process for controlling the concentration of a drug delivered intravenously to a patient as a function of the rate of infusion. A three-compartment pharmacokinetic (PK) model is used to determine the plasma drug concentration. Based upon the linear relationship between data pairs comprising a rate of infusion and a corresponding plasma drug concentration, an interpolated rate is determined by a microprocessor as a function of the specified plasma drug concentration. The actual infusion rate of the drug during successive time intervals is repetitively used to compute the plasma drug concentration at the end of each time interval. For each iteration, state variables from the previous computation are applied to determine the next interpolated infusion rate. The open-loop control method disclosed by Jacobs rapidly achieves the specified plasma drug concentration.
It will be apparent that controlling the administration of a plurality of drugs through a multi-channel drug infusion system using a PK model to predict the drug concentration and control the rate of infusion would provide significant advantages over the prior art control of a single channel. If desired, a different PK model could then be selectively applied to control the delivery of each different drug through each channel of a multi-channel drug delivery system, or the same model could be used for the control of each channel of the system.
A PK model control for a pump should have other features not common found in the prior art devices. For example, a physician should be able to selectively use a PK model for controlling drug administration to achieve a desired effect of a drug on the patient, instead of simply achieving a desired blood plasma drug concentration. PK models such as that disclosed in the above-cited Jacobs patent model the movement of drugs within the patient's body in terms of a plurality of compartments. Such a model can be adapted to include an effect compartment that hypothetically corresponds to the actual effect of the drug on a patient. By predicting the effect compartment drug level, it should be possible for the model to better control the rate of drug delivery. A physician is generally more concerned with obtaining the desired effect of the drug on the patient than with achieving some arbitrary blood plasma drug concentration.
At times, a physician may find it necessary to briefly alter the parameters of a drug therapy. If the drug administration is being controlled by a PK model, the physician may want to switch to a manual mode for a period of time, for example, to administer a bolus dose. It should thereafter be possible to switch back to the PK model controlled mode. Accordingly, it is important that the control for a pump that is used to administer drugs in accordance with a PK model be able to track the history of the drug administered and take into consideration changes that occur while the PK model controlled mode is interrupted by a manual controlled mode, when the PK model control mode resumes. Since the history of the blood plasma concentration must be retained to achieve this goal, the infusion system control should be able to display both historical and predicted blood plasma and compartment effect drug concentrations levels to medical personnel for each drug administered, during either model controlled or manual controlled modes of operation. The model should continue to track and display these parameters, even after the drug infusion has stopped, so long as the patient's case is active.